I would love it if Arabic was added to Duolingo!! I would really like to learn Arabic, and this is my favorite method of language learning.
Luis said the Arabic will be added Eventually, But never specified when.
I would also love to have Arabic on Duo ^^
While you wait, you many want to try Memrise's official Arabic course.
There are 7, i believe. I linked the 1st.
DigitalDialects has a few games for Arabic learning too.
I've heard a lot of people recommend Busuu, so here's their Arabic course..
Lastly, here is Benny Lewis' top Arabic resources list.
Hope i helped!